1. Field of the Invention
This invention relates to a shipping container for a nuclear fuel assembly and, in particular, to such a container for unirradiated nuclear fuel assemblies which have a plurality of fuel rods supported in a geometric array.
2. Background Information
In the shipping and storage of unirradiated nuclear reactor fuel elements and assemblies, which contain large quantities and/or enrichments of fissile material, U235, it is necessary to assure that criticality is avoided during normal use, as well as under potential accident conditions. For example, fuel shipping containers are licensed by the Nuclear Regulatory Commission (NRC) to ship specific maximum fuel enrichments (i.e., weights and weight-percent U235) for each fuel assembly design. In order for a new shipping container design to receive licensing approval, it must be demonstrated to the satisfaction of the NRC that the new container design will meet the requirements of the NRC rules and regulations, including those defined in 10 CFR 71. These requirements define the Maximum Credible Accident (MCA) that the shipping container and its internal support structures must endure in order to maintain the subcriticality of the fuel assembly housed therein.
U.S. Pat. No. 4,780,268, which is assigned to the assignee of the present invention, discloses a shipping container for transporting two conventional nuclear fuel assemblies having a square top nozzle, a square array of fuel rods and a square bottom nozzle. The container includes a support frame having a vertically extending section between the two fuel assemblies which sit side by side. Each fuel assembly is clamped to the support frame by clamping frames, which each have two pressure pads. This entire assembly is connected to the container by a shock mounting frame and plurality shockmountings. Sealed within the vertical section are at least two neutron absorber elements. A layer of rubber cork cushioning material separates the support frame and the vertical section from the fuel assemblies.
The top nozzle of each of the conventional fuel assemblies is held, along the longitudinal axis thereof, by jackposts with pressure pads that are tightened down to the square top nozzle at four places. The bottom nozzle of some of these conventional fuel assemblies has a chamfered end. These fuel assemblies are held, along the longitudinal axis thereof, by a bottom nozzle spacer which holds the chamfered end of the bottom nozzle.
This and an other shipping containers (e.g., RCC-4 for generally square cross-sectional geometry pressurized water reactor (PWR) fuel assemblies) used by the assignee of the present invention are described in Certificate of Compliance No. 5450, Docket No. 71-5450, US Nuclear Regulatory Commission, Division of Fuel Cycle and Material Safety, Office of Nuclear Material Safety and Safeguards, Washington, D.C. 20555.
U.S. Pat. No. 5,490,186, assigned to the assignee of this invention, describes a completely different nuclear fuel shipping container designed for hexagonal fuel and more particularly for fuel designed for Soviet style VVER reactors. Still, other shipping container configurations are required for boiling water reactor fuel.
There is a need therefore, for an improved shipping container for a nuclear fuel assembly that can be employed interchangeably with a number of nuclear reactor fuel assembly designs.
There is a need for such a fuel assembly shipping container that can accommodate a single assembly in a lightweight, durable and licensable design.
There is a further need for such a shipping container that can be readily loaded in both a horizontal or vertical orientation.
These and other objects are achieved by the individual fuel assembly containment system design of this invention to safely transport unirradiated nuclear fuel assemblies under normal and hypothetical accident conditions. The shipping container includes an elongated inner tubular liner having an axial dimension at least as long as the fuel assembly. The liner is preferably split in half along its axial dimension so that it can be separated like a clamshell for placement of the two halves of the liner around the fuel assembly. The external circumference of the liner is designed to be closely received within the interior of an overpack formed from an elongated tubular container having an axial dimension at least as long as the liner. Preferably, the wall of the tubular container is constructed from relatively thin shells of stainless steel coaxially positioned with close cell polyurethane disposed in between. Preferably, the inner shell includes boron-impregnated stainless steel.
The inner tubular liner enclosing the fuel assembly is sideably mounted within the tubular container overpack and the overpack is sealed at each end with end caps. The tubular container overpack preferably includes circumferential ribs that extend around the circumference of the tubular container at spaced axial locations, that enhance the circumferential rigidity of the overpack and form an attachment point for peripheral shock absorbing members.
An elongated external frame, preferably of the birdcage design, is sized to receive the tubular container within the external frame in spaced relationship with the frame. The frame is formed from axially spaced circumferential straps that are connected to circumferentially spaced, axially oriented support ribs that fixedly connect the straps to form the frame design. A plurality of shock absorbers are connected between certain of the straps and preferably at least two of the circumferential ribs extending around the tubular container, to isolate the tubular container from a substantial amount of any impact energy experienced by the frame should the external frame be impacted.